1. Technical Field
The present invention relates to a self-oscillating audio amplifier, and more particularly, to a self-oscillating audio amplifier capable of restraining distortion, and a method for restraining audio distortion of the self-oscillating audio amplifier.
2. Description of Related Art
Audio amplifiers play an important role in driving speakers, and common audio amplifiers include Class AB amplifiers and Class D amplifiers. The Class D amplifiers deliver an efficiency that is twice to five times higher than that of the Class AB amplifiers. Moreover, as compared to the Class AB amplifiers, power dissipation and required power of the Class D amplifiers are dramatically decreased.
Primarily, a Class D amplifier amplifies an audio signal by employing the pulse width modulation (PWM) technology to transform audio signals into an on/off status control signals for controlling the on/off status of power electronic components. Besides, an inductor-capacitor (LC) filter incorporated in the Class D amplifier itself can store sufficient energy in each operation period of the Class D amplifier, so as to avoid power dissipation at a load end (e.g., a speaker) of the Class D amplifier when the on/off status of power electronic components are being controlled.
According to the PWM technology employed in the Class D amplifier, triangular wave signals are generated through self-oscillation of an integrator or by means of a triangular wave generator, and then a comparison is made by a comparator to output PWM signals. As shown in FIG. 1 which is a circuit diagram of a conventional self-oscillating audio amplifier, the self-oscillating audio amplifier 100 mainly comprises: an audio signal 104, a clock switch 128, a clock input source 116, a sub-audio amplifier 106, a pre-driver 108, a transistor set 110, a filter inductor 112, a filter capacitor 114 and a speaker 120. Further, as shown in FIG. 2 which is a circuit diagram of the sub-audio amplifier 106 of the conventional self-oscillating audio amplifier, the sub-audio amplifier 106 mainly consists of a resistor 124, a capacitor 134, a comparator 122 and a comparator 132. The sub-audio amplifier 106 is configured to transform the audio signals into the triangular wave signals and, by means of the comparator 132, compares the triangular wave signals against an externally applied reference voltage VREF so that the comparator 132 can output pulse-modulated signals.
The conventional self-oscillating audio amplifier 100 as described above accomplishes the goals of audio amplification and pulse modulation by means of self-oscillation of the sub-audio amplifier 106. This allows for outputting the audio signals to the speaker 120 in a high efficiency and responding to the inputted audio signals through the feedback resistor. However, although the integrator circuit is able to self-oscillate in the absence of input audio signals to present less distortion, it is impossible to restrain distortion of the audio signals caused by a too high output power.
Accordingly, in view of the shortcomings of the conventional self-oscillating audio amplifiers, the inventor of the present invention has made great efforts to make improvement thereon and finally provides an improved self-oscillating audio amplifier and a method for restraining distortion of the improved self-oscillating audio amplifier.